This invention relates to a method of continuously casting stainless steel slabs which when hot rolled to an intermediate gauge have improved surface quality. More particularly, this invention relates to a method of continuously casting austenitic stainless steel with ultra low sulfur levels to improve the surface quality of hot-rolled band.
In the production of steel slabs, it is a customary practice to produce a melt of the desired steel composition by any of conventional means, including an electric furnace, a top-blown oxygen converter, or an argon-oxygen decarburization (AOD) vessel. The steel, which may be a stainless steel, in molten form is then transferred from the furnace to a transport ladle from which it is teemed into a flow-through continuous casting mold and apparatus. The steel is cooled within the continuous casting mold to form a slab having a solidified skin and a molten metal interior. This partially solidified casting is then passed through a series of support rolls and water-cooling sprays which serve to further solidify the casting so that it is completely solidified before it exits from the support rolls. The casting is then cut to desired lengths for further processing.
Generally, further processing includes hot rolling and cold rolling to final gauge. Typically, however, prior to the hot-rolling operation, the slab surface is conditioned by a surface removal operation, such as surface grinding, to remove oxides, scale, and surface defects which may be in the form of nonmetallic inclusions, as well as oscillation marks resulting from the continuous caster. In many cases, a failure to condition the slab prior to hot rolling will result in poor surface quality of the hot-rolled band and the cold-rolled final gauge product.
Whether or not the slab is conditioned, it may then be reheated or annealed in a furnace and hot rolled to an intermediate or final gauge. Conventionally, the hot-rolled band may be annealed and descaled, such as by shot blasting and pickling, and inspected for surface quality. If the hot-rolled band has good surface quality, the band may then be acceptable for its intended applications or may be further processed by cold rolling with or without intermediate anneals to a cold-rolled final gauge. If the hot-rolled band quality is unacceptable, the band may be ground or otherwise surface treated to improve the surface quality, or may be scrapped. The required slab surface conditioning operation prior to hot rolling and the conditioning of the hot-rolled band are labor-intensive operations and add considerably to the overall cost of production and lessen productivity by increasing yield losses.
Attempts have been made by others to improve the hot workability of stainless steels. The adverse effects of sulfur on hot workability are generally well known. For example, U.S. Pat. No. 4,007,038, assigned to the common Assignee of the present application, discloses providing good hot workability characteristics in a high molybdenum austenitic stainless steel by maintaining sulfur of less than 0.006%, together with additions of critical amounts of both calcium and cerium. The hot workability is characterized by reduced edge checks in the hot and cold finish strips.
It is desirable to improve the hot-rolled band surface quality of coils rolled from slabs which are continuously cast. More particularly, it is desirable to improve the hot-rolled band surface quality from slabs whether or not they are conditioned prior to the hot-rolling operation. It is desirable that any method for improving the hot-rolled band surface quality permit bypassing of the conditioning operation to reduce the overall cost of the final product. The method should result in continuously-cast slabs which result in hot-rolled band of improved surface quality whether or not the slabs are surface conditioned. It is further desirable to improve the hot-rolled band surface quality of the chromium-nickel and chromium-nickel-molybdenum grades of stainless steels which at the hot-rolled band stage appear to be more prone to metallurgical slivers and/or defects due to oscillation marks from the slab.